The factor Y comes from the Boardman equation which is an empirical equation which matches the Lame equation solution for the circumferential stress on the inside diameter of a cylinder. The circumferential stress on the inside of a cylinder under internal pressure is higher than the circumferential stress on the outside because of the effect of radial stress (=internal pressure on inside surface), poisson's effect, and the fact that meridional (axial) strain through the thickness of a cylinder must remain constant (increase in circumferential stress offsets axial strain caused by radial stress, both poisson's effects). The same equation is used in Section VIII Div 1 but the equation is recast based on inside diameter rather than outside diameter. The empirical relation with y=0.4 becomes less accurate for thick walled cylinders, and the equation cited by Prex corrects this. In the creep regime, the stress distribution through the wall becomes more constant, which is why the y factor is changed for temperatures in the creep regime. If you want more detail (or more long winded description), you can purchase the book I just published through ASME, Process Piping, The Complete Guide to ASME B31.3.